How Does Rescuer's Position Setting Impact Quality of Chest Compression: A Randomized Crossover Simulation Study on Unexperienced Clinicians

Background High-quality chest compression (CC) is the crux of survival for cardiac arrest patients. While, rescuers' position setting relative to patients during CC was unrecommended in the present guidelines. We aimed to assess the impact of position settings on CC quality during cardiopulmonary resuscitation (CPR) and to test the heterogeneity related to rescuers' characteristics. Methods We conducted randomized, crossover, simulation trials with clinical students unfamiliar with CPR. The participants received standard training on performing CC and were divided randomly into two groups. The two groups separately performed CC with standing and kneeling positions in turn, forming the crossover design. The trials were performed with standard manikin models. CC quality indicator data were recorded by the tracking and feedback system automatically. Result 156 participants finished at least one round of trial, with 126 participants finishing both rounds. Records for CC with kneeling and standing positions showed statistically significant differences in the correct rate, pause happening, average depth, and happening of over-depth compression. Regression analysis also implied that larger compression depths with the standing position were related to larger height and BMI of the participants. Conclusion When performing CC, the standing position will lead to lower CC quality by larger chance of pause happening and over-depth compression. In addition, compression depth gaps between CC with kneeling and standing position were related with rescuer characteristics including height and BMI, with a threshold effect.


Introduction
Te incidence and mortality of cardiac arrest are still very common [1,2].Clinical evidence and the guidelines hold that chest compression (CC) is the crux in series of cardiopulmonary resuscitation (CPR) operations.High-quality CC is associated with higher survival rate for patients with cardiac arrest [3][4][5].CC quality can be measured by a system of indicators including the compression depth, rate, pause happening, chest recoil, and compression fraction [6].Current guidelines recommend that a proper compression rate (100-120 times per minute), compression depth (5-6 centimeters), and lowest incidence of compression pause are basic components of the successful CPR [6,7].
In clinical practice, performing high-quality CC by hands is still a major challenge for rescuers.Tiredness is the enemy of proper CC [8].As a result, the guidelines suggested that the rescuers should be replaced in every two minutes [6,7].Rescuers' tiredness will difer according to their gender, age, and body mass index (BMI) [9].While some factors exogenous to the rescuers can also infuence the tiredness happening and afect CC quality [10].Diferent CPR methods and varied relative position settings between the rescuer and patient are also key infuencers [11,12].
As for position settings, out-of-hospital patients with cardiac arrest will always receive CC on the ground, while inhospital cardiac-arrest patients will more likely be chest compressed on fxed bed.Studies have shown that rescuer's relative position to the patient can infuence the efectiveness of CC during CPR [13].But the current guidelines from American Heart Association (AHA) [4] and European Resuscitation Council (ERC) [6] ofer no clear recommendation for the rescuer's position settings.
Tere are great theoretical and practical values to understand deeper on proper position settings for CC during CPR.Consequently, we in this article tried to explore how and why the position settings of rescuers performing CC can impact the quality of CC with cardiac arrest manikin simulation.

Study Setting and Ethics
Approval.Tis study was a single-center, randomized and crossover, observational simulation study.Te research has been approved by the Ethics Review Committee of Beijing Union Medical College Hospital (with approval number: I-22PJ814).Before enrolling into the study, each participant had read and signed the informed consents.Te research was carried out in accordance with requirements stated in the Helsinki Declaration of 1964 which was revised in 2013 [14].

Study Population.
We included clinical students on their third or fourth year during the program of medicine doctor for eight years with basic medical training but were new to CPR operation.Tey were enrolled by the whole classes, with good representativeness of young clinicians in China.Before the formal trials, all participants received training on basic life support (BLS) according to the guidelines of American Heart Association (AHA), including being ofered learning materials on CPR, lectures for 15 minutes on how to perform qualifed chest compression, and standard practices on manikin models for fve rounds with feedback about compression quality, followed by the BLS tests.When they had passed the tests, they were qualifed to participate into the formal trials [15].

Detailed Design.
We divided the qualifed participants into two groups randomly.With random drawing lots, those with odd numbers were allocated into the frst group and those with even numbers into the second group.In the frst round of the trial, participants in the frst group knelt beside the manikin on the ground (kneeling position setting) to perform CC, and participants in the second group stood beside the manikin which was on the bed of 60 centimeters high (standing position setting) when performing CC.Te CC lasted for 2 minutes as the guidelines recommended.Te participants in both groups were required to perform CC with the rate of 100-120 times per minute, the compression depth between 5 and 6 centimeters, and least pausing during CC, also as the guidelines recommended.
Ten, both groups of participants rested for 60 minutes after the frst round of trials.In the second round of trials, the frst group performed CC with the standing position and the second group with the kneeling position.Te trials at the second round were performed with the same time period and the same requirements as at the frst round.In this way, the random crossover design was realized with the same participant performing CC with both standing and kneeling positions in turn.
During the two rounds of trials, no feedback information on compression quality were ofered to the participants, letting them to fully simulate the practice by unexperienced rescuers.All the data on CC quality for trials in each round of every participant were automatically recorded by computer system linked to the manikins.

Outcome.
Te primary outcomes of the study were 8 kinds of CC quality indicators during CPR process, which will be compared between kneeling and standing position settings.Te secondary outcome was the relation between the rescuer's height or BMI and CC quality diferences with varied position settings.

Statistical Analysis.
In this article, categorical variables were presented with frequencies in percentage.Also, continuous variables were presented as the mean value with standard deviation (S.D.) if the variables conform to normal distribution.Otherwise, they were presented using the median with interquartile range (IQR).Te Shapiro-Wilk test was used to test whether the continuous variables conform to normal distribution.Categorical variables were compared between the records of CC performed with kneeling and standing positions using the Pearson Chisquared test or Fisher's exact test.Besides, we use Student's t-test for data with normal distribution and the Mann-Whitney U test for variables with non-normal distribution.Regression analyses including threshold regression were systematically utilized [16].
Te major statistical analyses and the best-ft line diagram of rescuer's height or BMI and CC depth gap were performed by STATA software (version 15.0).Bilateral P value less than 0.05 stood for statistically signifcant result.

Study Participants.
Initially, the study enrolled 156 clinical students and gave them BLS training according to AHA guidelines.Tose participants were divided into two groups randomly with 78 in each group.After the two rounds of crossover trials of CC with diferent positions, 282 qualifed records were collected.While only 126 participants fnished the both rounds of trials with 252 paired records.Te detailed fowchart of participants' keeping is in Figure 1.
In Table 1, we provided the basic characteristics of the participants.Based on Panel A, the average age was around 22, and 61 of them were male (29.1%).Te average height was 168.12 centimeters with the standard deviation (S.D.) of 7.63 centimeters, and the average weight was 60.85 kilograms with S.D. of 10.74 kilograms.Based on the height and weight of the participants, we obtained the BMI.Te average BMI was 21.42, with S.D. of 2.72.For the gender heterogeneity, both male and female participants share the similar average age because we enrolled them with whole classes as the units.

Emergency Medicine International
Males were taller and larger in weight than females, which showed signifcant diference (P-value <0.001).In addition, the males and females difer signifcantly on BMI as well.
Besides, as for 126 participants fnishing both rounds of the trials posted in Panel B, the statistic results of the participants' characteristics were similar.Note.All values were estimated with means (and with S.D. followed after "±").P values were ofered to tell the diferences between the male and female participants.
Emergency Medicine International 3

Te Quality of Chest Compressions.
We compared CC quality between diferent position settings in Table 2.We found that CC with the standing position had a lower correct rate but more probability of pause happening and was about 2.48 millimeters deeper compared with the kneeling position (P value � 0.019).CC with the standing position had a larger over-depth compression happening rate and a lower lessdepth compression happening rate (P value <0.001).While CC with varied position settings turned out to be similar on compression rate and similar chance of over-speed and lessspeed compression.Besides, we kept the participants fnishing both rounds of trials shown in Table A1 of Supplementary Materials, and the results stayed robust.Furthermore, we went on with the regression results for standing position's impact on CC quality compared with the kneeling position, with participants' characteristics including age, height, and BMI controlled.In Figure 2, the coefcients (varied dot shapes for varied dependent variables) and confdential interval (bars with 2 endpoints) were ofered.Te results showed a similar result with Table 2.
Ten, we went on to analyze whether there was heterogeneity between male and female.Based on Table 3, males had larger chances of correct compression but also deeper compression, as well as a larger over-depth compression rate.Compression depth of females with kneeling position was more likely below the minimum standard of correct CC of 5 centimeters, which might cause a lower correct rate.Te similar situation can be found when we kept 126 participants fnishing both rounds of trials, as shown in Table A2 in Supplementary Materials.
In addition, we performed regression analysis of standing position setting's impact on CC quality compared with kneeling position setting, with personal characteristics controlled (Figure 3).For gender heterogeneity, standing position setting could lead to a lower correct rate in females but larger compression depth in male.We anticipated that the heterogeneities between male and female were due to the variety on personal characteristics such as height and BMI.

Te Impact of Rescuer's Height and Weight on CC
Quality.In this part, we provided analysis to explore the height and BMI of rescuers' impact on CC quality diferences related to varied position settings, and especially whether threshold efect existed.Te analysis unit was every individual record.
We choose the correct rate and compression depth as dependent variables because male and female varied signifcantly in those two CC quality indicators based on previous analyses.Te threshold regression results are reported in Table 4. On the upper panel, we separately tested whether the standing position compared with the kneeling position would have diferent impacts on the correct rate for varied height and BMI ranges.Although there were some diferences, the threshold efects were not signifcant based on Bootstrap P values.On the lower panel, the dependent variables were compression depth.Based on Bootstrap P values, the threshold efect existed.With larger height over 165 cm, the participants tended to have larger compression depth with the standing position compared to the kneeling position.Also, participants with larger BMI over 20.32 would show larger compression depth with the standing position.So, the diferences between male and female might be due to the diferent rescuers' characteristics of height and BMI.
Enlightened by the fndings, we went on to test the participant height and BMI's impact on the gaps of compression depth with standing and with the kneeling positions by the same rescuer.Tanks to the crossover design, we could gather data on compression depth with kneeling and standing positions for the same rescuer.Subtracting the average compression depth with the kneeling position from the average compression depth with standing position for the same rescuer, we had the gap of compression depth.In the upper part of Figure 4, we ofered the best-ft line with rescuer's height as predictor for the gap of compression depth setting 165 cm as the threshold and found that one centimeter of height could lead to larger gaps of compression depth when the height of the rescuer was larger than 165 cm.In lower part of Figure 4, we analyzed the BMI as the threshold variable.One unit rise in BMI could cause the gap of compression depth by −3.21 if the BMI was smaller than 20.32 but causes little change if the BMI was larger than the threshold.

Discussion
For position setting in CC during CPR, rescuers' hand position [17], compression position on patients' body [18], and rescuers' relative position of standing or kneeling are all vital for CC quality.While rescuers' standing or kneeling choice is less explored under the latest guidelines and is our major focus.CC quality indices, like compression rate and depth, might always catch researchers' interests [17,19].While, impact of over-depth compression was always neglected, and was the interest of our study.Besides, studies on CC position setting were carried out with varied methods based on real-world data [18] or simulation design [20], applicable to diferent research situations.Among those methods, simulation trials on manikins providing steady trial environment and avoiding potential ethical problems was adopted in this study.
We included clinical students getting BLS trained for the frst time, ofering a homogenous and large-size participants sample.Te design excluded potential interferences caused by experienced rescuers who can intentionally adjust their compression operation [21].By simulating CC on manikins and comparing CC quality between kneeling and standing positions, we found that the standing position was more related to the deviating compression correctness, especially higher chance of pause happening and improper compression depth.Te fndings are important for clinical practice considering the potential harm that improper compression might cause [22,23].
Previous studies implied that the compression depth is smaller when performed with the patients on bed compare to on ground with the kneeling position [24] but neglected the infuence of rescuers individual characteristics.In this study, regression analyses showed that the compression 4 Emergency Medicine International depth diference between kneeling and standing positions was partly related to the rescuer's variety on height and BMI.Te taller the rescuer was, the larger the compression depth would happen with the standing position, showing a clear threshold efect.While for BMI, with larger BMI over 20.32, the gap between compression depth with standing and kneeling positions would be steady.
Besides, previous studies were performed under the direction of AHA CPR guidelines of 2010 which did not set the maximum value for compression depth [25,26].Some researchers have mentioned that the proper compression depth can vary according to the age ranges of the patients [27] and pointed out the potential harm of over-depth compression.Our study was based on the latest  Note.All values were estimated with means (and with S.D. followed after "±").P-values were ofered to tell the diferences between the kneeling and standing position settings.
Emergency Medicine International guidelines [6,7] and found that one major quality problem of CC was over-depth compression less explored.Furthermore, we ofered analysis on CC position setting choice during CPR in the simulation of practical situation, hopping to provide more enlightenment for clinical practice.Cho et al. studied how the height of bed can infuence the outcomes of CC [28], while we did not adjust the bed height according to the rescuer's height because the settled bed height was more approximate to the real-world clinical practice.Te bed adjustment process can cause delay for intime CPR.As a result, to explore the best choice for rescuer's position setting based on rescuer's individual characteristics such as height and BMI can be more valuable to ofer direct instruction for clinical practice.
Although we were confdent that our study can ofer some enlightenment, there were also limitations as well.First, this study was done with manikins instead of real patients considering the ethical reasons.In the real clinical situation, the position setting should be decided based on the actual environment, and the patients can vary greatly from each other, challenging the robustness of our fndings.Second, the participants in our trials all performed CC in  Note.All values were estimated with means (and with S.D. followed after "±").P values were ofered to tell the diferences among varied subsamples.Note.Coefcients of regressions for diferent models were ofered with 95% confdence interval (CI) in the parentheses followed.Bootstrap P values were ofered to tell whether the threshold efect existed.
Emergency Medicine International 2 minutes for both the kneeling and standing position settings, which is widely adopted with simulation trial of CC [20].While in clinical practice, the CC period can be longer and tiredness in those two position settings can be diferent, which was not fully considered yet.Tird, we enrolled the participants among clinical students instead of the experienced doctors and nurses.Even if the design can ofer more pure treatment related to position settings only, the strategy utilized can be diferent from clinical practice where the experience of emergency rescuers might infuence the CC quality [29].Also, some latest studies found that the proper CC depth depends on the patient characteristics like external anteroposterior diameter [30].CC depth can also be related to rescuer's handedness which is widely studied recently [19,31].As a result, whether the standing or kneeling position are more adoptable when considering patient individual characteristics and rescuer handedness calls for more explorations in the future.

Conclusions
When unexperienced rescuers perform CC during CPR, the position setting can infuence CC quality.CC with the standing position beside patient on bed was related to larger probability of pause happening (53.85%) and higher overdepth compression rate (29.51%).Te diference with varied position settings was related to rescuer's characteristics.With higher rescuer height, especially when the rescuers' height was larger than the threshold value of 165 cm, there was a bigger chance of over-depth compression for standing position setting.Consequently, the kneeling position, which can overcome the disturbance of rescuers' height, is more suggested for novice rescuers.All in all, for unexperienced rescuers performing CC, both proper position setting choice and proper training are important.

Figure 2 :
Figure 2: Regression results of standing position's impact on CC quality compared with the kneeling position.

Figure 3 :
Figure 3: Regression results of standing position's heterogeneous impacts on CC quality of male and female participants compared with the kneeling position.Note.Coefcients (varied dot shapes for varied dependent variables) and confdential interval (bars with 2 endpoints) were marked for regression analysis with varied CC quality indicators as dependent variables.

Figure 4 :
Figure 4: Relationship between rescuer's height/BMI and compression depth gap considering the threshold efect.

Table 1 :
Baseline characteristics of the participants.
Those finished CC in at least one round of trial (with 282 qualified records totally) CC in standing position CC in kneeling position Those who finished CC in both kneeling and standing positions (n=126, with 252 paired records totally) Cross-over with inter-group exchange Keep the participants finishing both rounds of trials Figure 1: Trial fowchart of the participants' keeping.

Table 2 :
Evaluation of CC quality for kneeling and standing position settings.

Table 3 :
Heterogeneity CC quality between male and female.

Table 4 :
Treshold regression with varied height and BMI ranges.